Who is The Morrigan, The Phantom Queen of Celtic Mythology?

A figure who appears as both a beautiful woman and a terrifying hag, a powerful goddess of war and a weaver of fate, a mistress of sovereignty and a harbinger of doom. She is known for her ability to shape-shift, to appear in different forms to different people, to grant blessings and curses with equal ease. Her stories are full of mystery and magic, of battles won and lost, of heroes and villains, of life and death. Who is this enigmatic figure, who embodies both light and darkness, both creation and destruction?